Create a square block extrusion in sculpt?
You can use multiple tools to achieve the effect, but I think the easiest is to use a stencil mapped texture with the shape of the window and a Layer brush. The height of the layer brush determines the depth of the effect. For best results, the sculpt plane should be "Area Plane" locked (ticking the lock icon next to the drop down menu).

Here is a rough picture with the settings: